1. Field of the Invention
The present invention relates to a misfire detection apparatus for an internal combustion engine capable of detecting combustion and misfiring in cylinders of the engine according to a change in the amount of ions generated by the combustion of an air fuel mixture.
2. Description of the Related Art
It is generally known that ions are generated when an air fuel mixture is burnt or combusted in cylinders (i.e., combustion chambers) of an internal combustion engine. Thus, using a probe arranged in each cylinder and impressed with a high voltage as a bias voltage, the ions thus generated can be observed as an ionic current whose magnitude corresponds to the amount of ions generated. That is, the occurrence of combustion or misfiring in all the cylinders can be individually detected or determined by detecting the presence or absence of such an ionic current.
However, soot might sometimes adhere to the electrodes of a spark plug installed in each cylinder as a result of combustion of the air fuel mixture depending upon the operating condition of the internal combustion engine.
FIG. 8 is an explanatory view that shows the operation of a conventional misfire detection apparatus for an internal combustion engine disclosed in Japanese Patent Laid-Open No. 2001-90647, and FIG. 9 is a block diagram showing the construction of the conventional misfire detection apparatus.
For instance, assuming that a bias voltage is 100 V and the insulation resistance of a spark plug with soot adhered thereto is 5 Mxcexa9, a leakage current of 20 xcexcA flows. As a result, a leakage current flows into an ionic current detection part while monotonously attenuating at a prescribed time constant in accordance with the impression of an ignition pulse IB, as shown in the FIG. 8. In addition, after the start of discharging of a spark plug, an ionic current generated by combustion of an air fuel mixture flows while being superposed on the leakage current that decreases monotonously and gradually according to a time constant CR which is determined by a high resistance of the sooted ignition plug and a capacitor component C of a bias circuit 2.
In FIG. 9, the bias circuit 2 of the misfire detection apparatus for an internal combustion engine impresses a bias voltage on a spark plug (not shown) installed in each cylinder for firing an air fuel mixture therein, and outputs an ionic current (pulse) superposed on a leakage current generated between the electrodes of the spark plug.
A mask circuit 5 masks pulses in the ionic current, which is output from the bias circuit 2 and waveform shaped with a fixed threshold, for a predetermined period of time thereby to cut or mask noise (e.g., ignition noise, etc.) in the ionic current (i.e., generate a masked signal).
A bottom value of the waveform-shaped ionic current after removal of the mask is held as a bottom hold value, based on which a threshold for determining the presence or absence of an ionic current is set.
After a prescribed mask time has elapsed, the ionic current output from the bias circuit 2 is input to a bottom hold waveform shaping circuit 20 where the level of the ionic current thus input is compared with the threshold which is set based on the bottom hold value. As a result of such a comparison, at the time when the level of the ionic current exceeds the threshold, the bottom hold waveform shaping circuit 20 determines that combustion has taken place, and outputs a combustion pulse.
Thus, the ionic current superposed on the leakage current is waveform shaped into the combustion pulse which is then output. Therefore, it is possible to detect combustion in an explosion cylinder irrespective of the magnitude of the leakage current.
With the conventional misfire detection apparatus for an internal combustion engine as described above, the ionic current superposed on the leakage current decreases monotonously with the passage of time, as shown in FIG. 10 for instance. Accordingly, there has been a problem in that in cases where there is no increasing period or range of the ionic current, as shown by a range t11 in FIG. 8, it is impossible to determine an occurrence of combustion or misfiring based on the bottom hold value.
The present invention is intended o solve the problem as referred to above, and has for its object to provide a misfire detection apparatus for an internal combustion engine which is capable of accurately determining an occurrence of combustion or misfiring in each cylinder of the engine even when an ionic current superposed on a leakage current decreases monotonously to provide no period or range in which the ionic current increases.
Bearing the above object in mind, the present invention resides in a misfire detection apparatus for an internal combustion engine which includes, an ionic current detector for detecting an ionic current signal corresponding to an ionic current generated when an air fuel mixture in a combustion chamber in each cylinder of the internal combustion engine is combusted, a gradient detector for detecting a first gradient of the ionic current signal at first detection intervals and a second gradient of the ionic current signal at second detection intervals longer than the first detection intervals, and a determiner for determining an occurrence of combustion or misfiring in the combustion chamber based on the first and second gradients of the ionic current signal. According to the above arrangement, combustion or misfiring can be detected in an accurate manner even in cases where a leakage current is generated with an ionic current decreasing monotonously in the absence of any increasing range of the ionic current.
The above and other objects, features and advantages of the present invention will become more readily apparent to those skilled in the art from the following detailed description of preferred embodiments of the present invention taken in conjunction with the accompanying drawings.